Appendix 3 Answers to Exercises

(3) Answers to Exercises – Physical Geology

The following are suggested answers to the exercises embedded in the various chapters of Physical Geology. The answers are in italics. Click on a chapter link to go to the answers for that chapter. (Answers to the chapter-end questions are provided in Appendix 2.)

Responses will vary, but your sample should look something like the one shown below. Granitic rocks are hard and strong and difficult to break. They are dominated by feldspar (this one has both white plagioclase and pink potassium feldspar), but almost all have some quartz (which looks glassy) and a few per cent of dark minerals, like the black amphibole in this one.

An example of a granitic rock [SE]

1.2 Plate motion during your lifetime – It depends where you live of course, but if you live anywhere in Canada and anywhere in the US east of the San Andreas fault, then you’re on the North America Plate, and that is moving towards the west at 2 to 2.5 cm/year. So if you’re around 20 years old, that plate has moved between 40 and 50 cm to the west in your lifetime.

1.3 Using geological time notation – 2.75 ka is 2,750 years, 0.93 Ga is 930,000,000 years or 930 million years, 14.2 Ma is 14,200,000 years or 14.2 million years.

1.4 Take a trip through geological time – 1) The oxygenation of the atmosphere started at around 2.5 Ga (2500 Ma). It was a catastrophe for many organisms because they could not survive the strong oxidizing effects of free oxygen. 2) We don’t really know the answer to this, but it’s not very long if you include insects, and there is evidence of insect damage to some of the earliest plants. 3) Plants on land allowed for animals on land, so without land plants, we wouldn’t be here.

3.1 Rock around the rock-cycle clock – Sedimentary rock is buried deeper to make metamorphic rock, the metamorphic rock is uplifted and during this process the material overhead is eroded so that it can be exposed at surface. The metamorphic rock is then eroded to make more sediments, which are deposited and then buried to make sedimentary rock. This would likely take at least 60 million years.

3.2 Making magma viscous – responses will vary

3.3 Rock types based on magma composition

SiO2

Al2O3

FeO

CaO

MgO

Na2O

K2O

Type?

55%

17%

5%

6%

3%

4%

3%

intermediate (although the SiO2 level is borderline, there is too little FeO, MgO and CaO to be mafic)

74%

14%

3%

3%

0.5%

5%

4%

felsic

47%

14%

8%

10%

8%

1%

2%

mafic

65%

14%

4%

5%

4%

3%

3%

intermediate (although the SiO2 level is borderline, there is too much MgO and CaO to be felsic)

3.4 Porphyritic minerals – a) only olivine phenocrysts, b) pyroxene and amphibole phenocrysts, along with plagioclase with a composition that is about half-way between the Ca-rich and the Na-rich end-members.

3.6 Proportions of ferromagnesian silicates–the approximate proportions are: 10%, 50%, 30% and 2%, and the corresponding rock names are granite, gabbro (although it’s right on the boundary between gabbro and diorite), diorite and granite.

3.7 Pluton problems – a is a stock, b is a dyke (it cuts across bedding and the granite), c is a sill (it is parallel to bedding), d is a dyke, e is a sill.

Chapter 4

4.1 How thick is the oceanic crust? – The magma available to create oceanic crust at this setting is approximately 10% of the volume of the 60 km thick part of the mantle from which it is derived, so the oceanic crust should be about 6 km thick.

4.2 Under pressure – no answer possible

4.3 Volcanoes and subduction – The volcanoes are between 200 and 300 km from the subduction boundary, about 250 km on average. If the subducting crust is descending at 40 km per 100 km inland, the depth to the Juan de Plate beneath these volcanoes is between 80 and 120 km, or 100 km on average.

4.4 Kilauea’s June27th lava flow – 1) The flow front advanced at a rate of about 160 m/day or just under 7 m/hour between June 27th and October 29th 2014. That doesn’t mean that the lava only flowed at rates of a few m/hour over that time. It likely flowed much faster (probably 10s to 100s of m/hour), but it advanced in fits and starts, and the advancing front changed locations many times. At other times the flow spread out across the area. 2) Between January 2015 and January 2016 the flow did not extend any further northeast towards Pahoa. Instead it spread out across the plain to the north of Pu’ u’ o’ o.

4.5 Volcanic Hazards in Squamish

Hazard

Risk

Tephra emission

Yes, but much of the tephra from a large eruption would extend up into the atmosphere, and would not affect Squamish.

Gas emission

Yes, There could be dangerous amounts of sulphurous or acidic gases flowing down the mountainside into Squamish.

Pyroclastic density current

Yes, a pyroclastic density current that flows down the western or southwestern sides of Garibaldi could easily reach Squamish.

Pyroclastic fall

Yes, in the later stages of a large eruption some tephra (or pyroclastic fragments) cold rain down on Squamish

Lahar

Yes, Squamish is definitely at risk from a lahar on the western side of the mountain. The risk would be increased if the eruption takes place in winter or spring when the amount of snow is at a maximum.

Sector collapse

Yes, this is possible. The western side of Mt. Garibaldi has already collapsed several times since the last glaciation.

Lava flow

Yes, Squamish is at risk from lava that flows on the southern and western sides of the mountain. There is a Pleistocene-aged lava flow clearly evident in the photograph. It flowed down the southern flank, and then turned west towards where Squamish is situated today.

4.6 Volcano alert – The most important tools for monitoring volcanoes are seismometers, and while there is a good network of seismometers in southwestern BC, there are not enough in close proximity to Mt. Garibaldi to be able to accurately define the locations and depths of earthquakes around the volcano. So the first project would be to establish about 5 additional seismic stations in the Squamish region. They don’t have to be right on the mountain, but can be placed near to existing roads and highways in the area. They need to be secured to bedrock. Every effort should be made to have them located on all sides of the mountain. The second project would be to establish some means of measuring deformation of the mountain itself. This could be done with tiltmeters or GPS stations, but GPS would be better. The GPS receivers have to be placed on the flanks of the mountain, and they also have to be installed right on bedrock. That could be a real challenge in winter or spring, when there is lots of snow. While this work is going on, we should charter a helicopter to fly around the mountain to see if there is any sign of eruptive activity or melting snow, and to look for convenient places to install GPS stations. We may want to land in a few different places.

There isn’t a lot that we can say to the public at this stage, except that this sudden increase in seismic activity could mean that Garibaldi is getting ready to erupt, that the Geological Survey and all emergency measures organizations are working together on it, and that residents of the Squamish area, and anyone using highway 99, should keep listening to local radio stations for further updates. We could also establish a system to send out alerts via text message.

4.7 Volcanoes down under – We would expect to see composite volcanoes on the North Island, some 200 to 300 km inland (northwest) from the Kermadec Trench, and within the ocean along the same trend to the northeast of NZ. There is also the potential for composite volcanism to the south of the South Island, east of the Macquarrie fault zone, although there appears to be some doubt about whether subduction is actually taking place in this region.

Chapter 5

5.1 Mechanical weathering – see below

Examples of mechanical weathering [SE]

5.2 Chemical weathering

Chemical change

Process

Pyrite to hematite

oxidation

Calcite to calcium and bicarbonate ions

dissolution

Feldspar to clay

hydrolysis

Olivine to serpentine

hydrolysis

Pyroxene to iron oxide

oxidation

5.3 Describe the weathering origins of sands

Sand description

Possible processes

Fragments of coral etc. from a shallow water area near to a reef in Belize

Reefs are constantly being eroded by ocean waves, and the fragments are washed inshore by currents and then further eroded by wave action.

Angular quartz and rock fragments from a glacial stream deposit near Osoyoos

Quartz-bearing rocks have been eroded and transported by a glacier. The fragments may have been moved a short distance by a stream, but not enough to produce rounding.

Rounded grains of olivine and volcanic glass from a beach in Hawaii

The olivine and glass grains are eroded by waves from volcanic rock and then thoroughly rounded by waves on the beach

5.4 The soils of Canada

Soil type

Distribution

Explanation

Chernozem

Southern prairies

These are dry-climate soils developed on grasslands

Luvisol

Northern prairies and BC interior

Soils developed on sedimentary rocks in cool moist climates

Podsol

Mountainous parts of BC and large parts of northern Ontario and Quebec

7.1 How long did it take – It might have taken in the order of 20 to 25 million years for these garnets to form, but even more time is needed than that to produce the rock because we have to account for the sedimentary process and then burial and lithification and then deeper burial to reach metamorphic environment – several tens of millions more years.

7.2 Naming metamorphic rocks

Rock Description

Name

A rock with visible minerals of mica and with small crystals of andalusite. The mica crystals are consistently parallel to one another.

Schist or (preferably) Mica-andalusite schist

A very hard rock with a granular appearance and a glassy lustre. There is no evidence of foliation.

Probably quartzite

A fine-grained rock that splits into wavy sheets. The surfaces of the sheets have a sheen to them.

Phyllite

A rock that is dominated by aligned crystals of amphibole.

Amphibolite

7.3 Metamorphic Rocks in Areas with Higher Geothermal Gradients

Metamorphic Rock Type

Depth (km)

Slate

2 to 5

Phyllite

5 to 8

Schist

8 to 12

Gneiss

12 to 17

Migmatite

17 to 25

7.4 Scottish Metamorphic Zones

Metamorphic zones in southern Scotland [SE]

7.5 Contact metamorphism and metasomatism

Contact metamorphic rocks [SE]

Chapter 8

8.1 Cross-Cutting Relationships [SE]

Relative ages: 2: oldest: 3: middle, 1: youngest [SE]

8.2 Dating Rocks Using Index Fossils

Dating from overlapping fossils [SE]

Probable age: 92.6 to 92.7 Ma. If M. subhercynius was not present the interpreted age range would be 92.6 to 92.9 Ma

8.3 Isotopic Dating

Isotopic dating [SE]

With a ratio of 0.91 the age is 175 Ma (red dotted line)

8.4 Magnetic Dating – The possible age ranges are 3.05 to 3.12 Ma and 1.78 to 2.00 Ma

Dating based on magnetic-reversal chronology [SE]

8.5 What Happened on Your Birthday? – Answers will vary.

Chapter 9

9.1 How Soon Will Seismic Waves Get Here? – Times shown for velocity of 5 km/s.

Location/distance

Time (s)

Location/distance

Time (s)

Location/distance

Time (s)

Nanaimo (120 km)

24 s

Surrey (200 km)

40 s

Kamloops (390 km)

78 s

9.2 Liquid Cores in Other Planets

S-wave shadow zones used to determine the extent of a liquid core [SE]

9.3 What Does Your Magnetic Dip Meter Tell You?

Vertical orientation

General location

Vertical orientation

General location

Straight down

North pole

Up at a shallow angle

Southern hemisphere, near the equator

Down at a steep angle

Northern hemisphere, near the pole

Parallel to the ground

Equator

Exercise 9.4 Rock Density and Isostasy

Rock densities [SE]

Chapter 10

10.1 Fitting the Continents Together

Pangea [SE]

10.2 Volcanoes and the Rate of Plate Motion

Pacific Plate rates of motion [SE]

10.3 Paper Transform Fault Model – no answer possible

10.4 A Different Type of Transform Fault

Juan de Fuca and Explorer Plates [SE]

The Juan de Fuca Plate is moving faster than the Explorer Plate, which means that the Juan de Fuca Plate is sliding past the Explorer Plate. There is side-by-side relative motion on this plate boundary, and that makes it a transform fault.

10.5 Getting to Know the Plates and Their Boundaries [SE]

The extents of the Earth’s major plates [SE]

Chapter 11

11.1 Earthquakes in British Columbia

[SE from Earthquakes Canada at http://www.earthquakescanada.nrcan.gc.ca/index-en.php]

Most of the earthquakes between the Juan de Fuca (JDF) and Explorer Plates are related to transform motion along that plate boundary,

The string of small earthquakes adjacent to Haida Gwaii are likely aftershocks of the 2012 M7.7 earthquake in that area.

Most of the earthquakes around Vancouver Island (V.I.) are related to deformation of the North America Plate continental crust by compression along the subduction zone.

Earthquakes that are probably caused by fracking are enclosed within a red circle on the map.

11.2 Moment Magnitude Estimates from Earthquake Parameters

Length (km)

Width (km)

Displacement (m)

Comments

MW?

60

15

4

The 1946 Vancouver Island earthquake

7.3

0.4

0.2

.5

The small Vancouver Island earthquake shown in Figure 11.13

4.0

20

8

4

The 2001 Nisqually earthquake described in Exercise 11.3

6.8

1,100

120

10

The 2004 Indian Ocean earthquake

9.0

30

11

4

The 2010 Haiti earthquake

7.0

The largest recorded earthquake had a magnitude of 9.5. Could there be a 10?

A possible solution is 2500 km long and 300 km wide with 55 m of displacement. (Other solutions are possible.) These are unreasonable numbers because subduction zones don’t tend to fail over that length (typically not much more than 1200 km), rupture zones cannot be that wide because that takes us into the asthenosphere, and displacements are never likely to be that great.

11.3 Estimating Intensity from Personal Observations

Building Type

Floor

Shaking Felt

Lasted (seconds)

Description of Motion

Intensity?

House

1

no

10

Heard a large rumble lasting not even 10 s, mirror swayed

II

House

2

moderate

60

Candles, pictures & CDs on bookshelf moved, towels fell off racks

IV

House

1

no

Pots hanging over stove moved and crashed together

III

House

1

weak

Rolling feeling with a sudden stop, picture fell off mantle, chair moved

11.5 Is Your Local School on the Seismic Upgrade List? – answers will vary

Chapter 12

12.1 Folding style

In order to help with the interpretation, one of the beds has been traced (in yellow) on the diagram below, and two of the fold axes have been shown (in pink). These folds are symmetrical, and although they are tight they are not isoclinal. They are overturned.

Folded rocks (in yellow) and fold axes (pink) [SE]

12.2 Types of faults

Top left: a normal fault, implying extension

Top right: A reverse fault, compression

Bottom left: a series of normal faults, extension

Bottom right: a right lateral fault (implies that there is shearing, but it is not possible to say if there is extension or compression)

12.3 Putting strike and dip on a map

See map below for strike and dip symbols. Relative ages, from youngest to oldest:

dyke (youngest)

fault

layer g (although this layer isn’t intersected by the fault or the dyke so it is not possible to know that it is older based on the information available)

layer f

layer e

layer d

layer c

layer b

layer a (oldest)

Vertical cross section (above), map view (below) [SE]

Chapter 13

13.1 How Long Does Water Stay in the Atmosphere?

The volume of the oceans is 1,338,000,000 km3 and the flux rate is approximately the same (1,580 km3/day).

What is the average residence time of a water molecule in the ocean?

1,338,000,000/1,580 = 846,835 days average residence time for water in the ocean (or 2320 years)

13.2 The Effect of a Dam on Base Level

How does the formation of a reservoir affect the stream where it enters the reservoir, and what happens to the sediment it was carrying?

The velocity of the streams slows to zero and most of the sediment is deposited quickly.

The water leaving the dam has no sediment in it. How does this affect the stream below the dam?

With nothing to deposit, the water below the dam can only erode, so there will be enhanced erosion below the dam.

13.3 Understanding the Hjulström-Sundborg Diagram

[SE]

A fine sand grain (0.1 mm) is resting on the bottom of a stream bed.

(a) What stream velocity will it take to get that sand grain into suspension? ~20 cm/s

(b) Once the particle is in suspension, the velocity starts to drop. At what velocity will it finally come back to rest on the stream bed? ~1 cm/s

(a) What size of particles can be eroded at 10 cm/s? No particles, of any size, will be eroded at 10 cm/s, although particles smaller than 1 mm that are already in suspension will stay in suspension.

(b) What is the largest particle that, once already in suspension, will remain in suspension at c0 cm/s? A 1 mm diameter particle should remain in suspension at 10 cm/s.

13.4 Determining Stream Gradients

Gradients of Priest Creek (in red) [SE]

The length of the creek between 1,600 m and 1,300 m elevation is 2.4 km, so the gradient is 300/2.4 = 125 m/km.

Use the scale bar to estimate the distance between 1,300 m and 600 m and then calculate that gradient. 5.2 km, with a gradient of 700/5.2 = 134 m km

Estimate the gradient between 600 and 400 m. 3.6 km, with a gradient of 200/3.6 = 56m /km

Estimate the gradient between 400 m on Priest Creek and the point where Mission Creek enters Okanagan Lake. 4 km, with a gradient of 60/4.0 = 15 m/km

13.5 Flood Probability on the Bow River

Calculate the recurrence interval for the second largest flood (1932, 1,520 m3/s). Ri = 96/2 = 48 years

What is the probability that a flood of 1,520 m3/s will happen next year? 1/48 = 0.02 or 2%

Examine the 100-year trend for floods on the Bow River. If you ignore the major floods (the labelled ones), what is the general trend of peak discharges over that time? In general the peak discharges are getting lower (from an average of around 400 m3/s in 1915 to an average of about 300 m3/s in 2015)

Chapter 14

14.1 How Long Will It Take?

i = (37-21)/80 = 0.2, V= 0.0002 x 0.2 = 0.00004 m/s. At that rate it will take 2,000,000 s for the groundwater to flow from the gas station to the stream. That is 555 hours, or 23 days.

14.2 Cone of depression

The cone of depression increases the gradient of the water table in the area around the well. That should increase the rate at which water flows towards the well.

14.3 What is your water table doing?

[BC Ministry of the Environment at http://www.env.gov.bc.ca/wsd/data_searches/obswell/map/]

The water-level for a random observation well in BC is shown above. The water table is slowly rising at this location. Since 2004 the lowest water level has risen from just above 4 m below surface to around 3.6 m above surface and the highest level has risen from around 0.3 m below surface to nearly at surface (0 m). Prior to 2004, where the points are not joined with lines, the trend appears to be similar.

14.4 What goes on at your landfill? – Responses will vary

14.5 Finding a leaking UST in your community – Responses will vary

14.6 Manipulating a Contaminant Plume

What could you do at wells A and C to prevent this? Explain and use the diagram below to illustrate the expected changes to the water table and the movement of the plume.

Implications of pumping water from wells B and C and injecting water into well A [SE]

Possible Answer: Injection into well A will cause water table to rise there (like the reverse of a cone of depression), thus reversing flow direction to the right of well A and moving the plume towards Well B. Extraction from Wells B and C will cause cones of depression and help to reverse the flow and pull the plume back from the stream. Both wells B and C may receive contaminants and so the water from both may need treatment.

Chapter 15

15.1 Sand and water – responses will vary

15.2 Classifying Slope Failures

[SE]

15.3 How Much Does a House Weigh and Can It Contribute to a Slope Failure?

A typical 150 m2 (approximately 1,600 ft2) wood-frame house with a basement and a concrete foundation weighs about 145 t (metric tonnes). But most houses are built on foundations that are excavated into the ground. This involves digging a hole and taking some material away, so we need to subtract what that excavated material weighs. Assuming our 150 m2 house required an excavation that was 15 m by 11 m by 1 m deep, that’s 165 m3 of “dirt,” which typically has a density of about 1.6 t per m3.

165 m3 of excavated soil x 1.6 t/m3 = 264 t – thus the excavated material weighs about 1.8 times as much as the house. In this case weight has been removed from the slope by building the house.

Chapter 16

16.1 Pleistocene Glacials and Interglacials

Describe the nature of temperature change that followed each of these glacial periods.

In each case the temperature drops slowly building to a peak of glaciation, and then each of the glacial periods is followed by a very rapid increase in temperature.

The current interglacial (Holocene) is marked with an H. Point out the previous five interglacial periods.

The previous 5 interglacials are labelled 1 to 5 on the diagram below. Interglacial 2 had two distinct warm episodes.

[SE after http://en.wikipedia.org/wiki/Mount_Assiniboine#/media/File:Mount_Assiniboine_Sunburst_Lake.jpg]

16.4 Identify Glacial depositional environments

[SE after USGS at http://water.usgs.gov/edu/gallery/glacier-satellite.html

Chapter 17

17.1 Wave Height versus Length

This table shows the typical amplitudes and wavelengths of waves generated under different wind conditions. The steepness of a wave can be determined from these numbers and is related to the ratio: amplitude/wavelength.

1. Calculate these ratios for the waves shown. The first one is done for you.

2. How would these ratios change with increasing distance from the wind that produced the waves?

Amplitude

Wavelength

Ratio

m

m

ampl./length

0.27

8.5

0.03

1.5

33.8

0.04

4.1

76.5

0.05

8.5

136

0.06

14.8

212

0.07

Within increasing distance from the source the wave heights would gradually decrease and so the ratios would decrease.

17.2 Wave Refraction

[SE]

17.3 Beach Forms

Barrier islands could from if this was a low-relief coast with an abundant supply of sediment from large rivers.

Possible locations of coastal deposits [SE]

17.4 A Holocene Uplifted Shore

The melting of glacial ice around the world at the end of the last glaciation (between 14 and 8 ka – see Figure 17.25) led to relatively rapid sea-level rise (by a total of approximately 125 m) which resulted in this area being submerged. That was a eustatic process. In response to the loss of ice in this region of coastal British Columbia there was a slower isostatic rebound of the crust, which is why this area is now back up above sea level.

17.5 Crescent Beach Groynes

[SE]

Chapter 1818.1 Visualizing sea-floor topography

1) see map, below

2) This is the area between the southern tip of South America (Cape Horn) and the Antarctic Peninsula. The body of water between the two is the Drake Passage.

The short-term climate impact was significant cooling because the dust (and sulphate aerosols) would have blocked incoming sunlight. This effect may have lasted for several years, but its intensity would have decreased over time.

The longer-term impact would have been warming caused by the greenhouse effect of the carbon dioxide.

19.2 Albedo Implications of Forest Harvesting

Clear-cutting (or any logging activity) leads to a net increase in albedo, so the albedo-only impact is cooling.

19.3 What Does Radiative Forcing Tell Us?

Using the ΔT = ΔF * 0.8 equation the expected temperatures for 2011, 1980 and 1950 compared with the estimated 13.4 C in 1750 should be:

Based on this reasoning the estimated temperature for 1950 is13.9˚ C (which is close to the actual of 14.0 ˚ C), while that for 1980 is 14.4˚ C, which is well above the actual of 14.2˚ C. It’s also clear that we didn’t reach 15.2˚ C by 2011, because even in the hottest year so far (2015) the global average temperature was only 14.8˚ C.

So while the ΔT = ΔF * 0.8 equation is useful, it appears to overestimate the temperature, probably because it takes some time (years to decades) for the climate to catch up to the forcing.

19.4 Rainfall and ENSO

Describe the relationship between ENSO and precipitation in B.C.’s southern interior.

As shown on the diagram below, there are some examples where a strong ENSO signal corresponds with very strong precipitation in the interior (and on the coast as well). The two strongest El Niños (1983 and 1998) shown correspond with the highest recorded precipitation levels in Penticton. Some other strong El Niños (1958 and 1973) are associated with strong precipitation within 6 months of the ENSO peak, but others show a negative correlation between ENSO and rainfall (marked with “?”).

The Mesoproterozoic quartzite phyllite schist of Tasmania may correlate with the Purcell rocks of Canada. The main difference is that while the Tasmanian rocks are metamorphosed, the Purcell rocks are generally unmetamorphosed.

21.3 What Is Vancouver Island Made Of?

1) Less than 10% of Vancouver Island is Paleozoic (the Devonian volcanic rocks – Dv)

2) The most common rock type is the Triassic Karmutsen Volcanic rock (basalt – Tv). The most common rocks by age are the Mesozoic rocks (Jurassic volcanic, Jurassic granite and Triassic volcanic)

[SE]

21.4 Dinosaur Country?

This Cretaceous Dinosaur Park Formation sandstone is clearly cross-bedded implying that it was deposited in a stream environment.

21.5 The Volume of the Paskapoo Formation

1) The 60,000 km2 area of source rock would have to have been eroded to a depth of 750 m to create 45,000 km3 of sediment